Conventionally, as a device for supporting a crankshaft, a camshaft, a balance shaft, a rocker shaft and the like for a car engine, a split type roller bearing is used in general. Since the roller bearing has high load capacity, it is suitable for a bearing used under high load circumstances.
However, as a fuel-efficient car is increasingly demanded in view of environment, a needle roller bearing is used instead of the roller bearing recently in some cases. As compared with the roller bearing, although the needle roller bearing is low in load capacity, since its friction resistance is low at the time of rotating, its rotation torque or a feeding amount of lubricant to a supporting part can be reduced.
However, it is difficult to incorporate the normal needle roller bearing as the supporting device for the above shafts in structure. FIG. 1 shows a general car crankshaft. As can be clear from FIG. 1, a needle roller bearing to support a crank pin 2 of a crankshaft 1 cannot be press-fitted in the axial direction. Therefore, as the supporting device for the above shafts, a needle roller bearing only comprising rollers and a split type cage is used. The needle roller bearing having such constitution includes a two-split type in which a cage is split at two positions in a circumferential direction and a one-split type in which a cage is split at one position in a circumferential direction. In addition, although the split type cage is formed of an iron material or a resin material in general, it is known that the one-split type cage is easily formed of the resin material as compared with the iron material.
Furthermore, there is a needle roller bearing which comprises an outer ring, a plurality of rollers arranged along a circumferential direction so as to roll on an inner diameter surface of the outer ring, and a cage holding the rollers (Japanese Patent No. 3073937, for example). According to such needle roller bearing provided with the outer ring, since the roller is in contact with the outer ring linearly, high load capacity and high rigidity can be provided despite its bearing projected area is small. Therefore, this type of needle roller bearing is widely used in various fields including a car. The outer ring in the above case includes a solid outer ring manufactured by a machining process, and a shell type outer ring manufactured by a pressing process.
U.S. Pat. No. 1,921,488 discloses a needle roller bearing in which a cage is a split type and an outer ring is also a split type (half shells). FIG. 2 is a view showing the outer ring disclosed in U.S. Pat. No. 1,921,488. As shown in FIG. 2, the outer ring of the needle roller bearing formed of a cold rolled steel plate (SPC material) and the like is split by a split line 4c extending in an axial direction of the bearing and has two outer ring members 4a and 4b. Thus, it can be incorporated in a crank pin.
FIG. 3 is a view showing a needle roller bearing 6 disclosed in Japanese National Publication No. 2002-525533 of International Application. The needle roller bearing 6 supports a shaft 5 and it comprises an outer ring 7, a plurality of needle rollers 8 arranged on an orbit surface of the outer ring 7 so that they can roll thereon, and a cage 9 holding the needle rollers 8. According to the needle roller bearing 6, the cage 9 is prevented from being shifted in an axial direction by reducing a diameter at both ends of the bearing of the shaft 5 so that the cage 9 protrudes inward along a diameter.
According to the split type needle roller bearing disclosed in the Japanese Patent No. 3073937 or the U.S. Pat. No. 1,921,488, the bearing is disassembled into the outer ring and the rollers with the cage or into the outer ring, the cage and the rollers during transportation to a user in some cases. In this case, they have to be assembled again on the user side carefully. In addition, the disassembled bearing part could be lost.
According to the needle roller bearing disclosed in the Japanese National Publication No. 2002-525533 of International Application, since a structure for axial positioning is provided between the cage and the outer ring, or between the cage and the shaft, the bearing parts can be prevented from being disassembled during transportation. However, since a sectional configuration of the cage is specific, a configuration of an injection mold of the cage and steps for molding the cage become complicated and its cost is high.
Since load is deflected to a predetermined direction in the crankshaft 1 shown in FIG. 1 and the like during rotation, the outer ring of the bearing comprises a region in which the load is concentrated (referred to as the “load region” hereinafter), and a region in which great load is not applied (referred to as the “non-load region” hereinafter). Thus, when the outer ring of the needle roller bearing shown in FIG. 2 is set in the shaft, the boundary 4c of the outer ring members 4a and 4b is to be positioned in the non-load region.
However, since the outer ring members 4a and 4b are only fixed by being fit in a housing, when the outer ring members 4a and 4b receive load during the rotation, they could be rotated and displaced in the housing. At this time, when the boundary 4c of the outer ring members 4a and 4b is moved to the load region, a rotation defect of the needle roller could be caused or the outer ring members 4a and 4b could be damaged and the like.
In addition, according to the needle roller bearing 6 shown in FIG. 3 and the like, since a process for the shaft 5 to prevent the cage 9 from being shifted in the axial direction is needed, its processing cost is increased. Furthermore, in a case where cage controlling means 9a is too small, when large axial load is applied, the cage 9 could fall off. Meanwhile, in a case where the cage controlling means 9a is too strong, it could interfere in the shaft 5.
Furthermore, since the cool rolled steel plate (SPC material) used in the outer ring members 4a and 4b contains as low as 0.04% to 0.15% by weight of carbon, workability is high but quenching ability is low. Thus, in order to secure hardness (Hv 653 or more) required for the surface of the outer ring members 4a and 4b on which the rollers roll, a carburized nitriding process is needed. However, an equipment of the carburized nitriding process is expensive as compared with other heat treatment equipments, and a deep carburized layer is not formed in the cool rolled steel plate having a low carbon content, so that its surface could be peeled off and the like while the bearing is used.